Positive nuclear charge was explained by protons, but many nuclei weighed far more than their proton count. Rutherford had suggested a neutral particle with roughly the proton’s mass.
In 1930, radiation produced by bombarding beryllium knocked fast protons out of paraffin. It was initially described as unusually energetic gamma radiation. James Chadwick repeated the experiment with several targets and compared the collision energies.
A massless photon would need implausible energy to launch the observed protons. A neutral particle with nearly a proton’s mass explained them through ordinary elastic collision.
The neutron filled the nucleus’s missing mass and clarified isotopes: the same element can keep its proton count while changing its neutron count.
Chadwick did not see a neutron directly. He identified it by the motion left in other bodies. Much of particle physics works that way: invisible actors are known by the conservation laws written into their debris.
